As well known, circuit breakers used as electric switches for electrical circuits are classified into Earth Leakage Circuit Breakers (ELCBs) used to prevent electric shocks from passing through human bodies and fires from occurring due to leakage, and No Fuse Breakers (NFBs) used to cut off an excessive amount of current due to a short circuit and the overflow of current caused by excessive usage of a load (a device or a machine).
Of the two, the ELCBs are current-operated breakers employing a method of detecting current and cutting off a circuit when current leaking to earth is generated due to leakage at a load. The ELCBs use a Zero-phase Current Transformer (ZCT) as a detection device. An example of the ELCBs is illustrated in FIG. 1.
Referring to FIG. 1, a conventional ELCB includes a switching unit 10 for automatically cutting off the supply of power from an electrical circuit under the operation control of a control unit 50; a zero-phase current transformer 20 for detecting the leakage current of the electrical circuit; an amplifier for amplifying voltage of the leakage current received from the zero-phase current transformer 20; a detector 40 for rectifying and smoothing the leakage current received from the amplifier 30 and generating leakage voltage; the control unit 50 including a reference voltage generator 52 for generating reference voltage for determination of leakage and a comparator 51 for determining whether leakage occurs by comparing the leakage voltage from the detector 40 with the reference voltage from the reference voltage generator 52 and automatically controlling the operation of the switching unit 10; and a power supply controller 60 for controlling the supply of power to the respective components from the electrical circuit.
The operation of the ELCB is schematically described below.
When the switching unit 10 is turned on, current flows from a power source to a load. In this case, when the electrical circuit is in a normal state, magnetic fluxes generated in the zero-phase current transformer 20 are offset with each other, so that leakage current due to electromagnetic induction is not detected. In contrast, when the electrical circuit is in an abnormal state, a difference between magnetic fluxes, corresponding to leakage or over-load, occurs, so that leakage current corresponding to the difference between magnetic fluxes due to electromagnetic induction is output from the zero-phase current transformer 20.
If the electrical circuit is in an abnormal state, the leakage current detected by the zero-phase current transformer 20 is amplified by the amplifier 30 and is provided to the detector 40.
The detector 40 rectifies and smoothes the leakage current received from the amplifier 30, thereby generating leakage voltage. The leakage current received from the amplifier 30 is alternating current, and the leakage voltage obtained through the rectification and the smoothing is average voltage of the leakage current, which is direct current.
The reference voltage generator 52 generates a reference voltage in order to determine whether leakage occurs. The reference voltage is direct current voltage, and, if required, may vary.
In the above-described state, the comparator 51 compares the leakage voltage from the detector 40 with the reference voltage from the reference voltage generator 52, determines whether leakage occurs, and automatically controls the operation of the switching unit 10. The switching unit 10 is controlled by the comparator 51 and automatically cuts off the supply of power from an electrical circuit.
However, such a conventional ELCB employs a method of comparing the leakage voltage of total leakage current detected by the zero-phase current transformer 20 with the reference voltage and determining whether leakage from an electrical circuit occurs, so that problems occur in that the malfunction of the ELCB occurs and the improvement of safety is restricted.
In greater detail, the leakage current detected by the zero-phase current transformer 20 is leakage current in which the leakage current due to leakage resistance and the leakage current due to a condenser, which constitutes a load, are added to each other without distinction. For reference, the condenser is installed on the ground line of a load in order to prevent noise such as Electro Magnetic Interference (EMI) or Electro Magnetic Compatibility (EMC). The leakage current due to the condenser does not much affect the occurrence of an electric shock on a human body or the occurrence of a fire due to leakage.
As a result, when the reference voltage of the reference voltage generator is reduced without taking the leakage current due to the condenser into account, the ELCE reacts sensitively, thereby ensuring safety. However, there is a problem in that the ELCB frequently operates even when leakage does not occur. In contrast, when the reference voltage of the reference voltage generator is appropriately increased with the leakage current due to the condenser sufficiently taken into account, the malfunction of the ELCB due to the leakage current of the condenser is reduced. However, the ELCB reacts insensitively, so that a problem occurs in that safety is not ensured.
Conventionally, in the determination of reference voltage, the reference voltage is determined with the leakage current due to a condenser taken into account so as to ensure safety against leakage, while preventing an ELCB from unnecessarily operating.
Therefore, in the conventional art, there are limitations in the prevention of the malfunction of an ELCB and the improvement of safety.